Intra prediction exploits spatial correlation within a picture or within a picture region. In order to improve coding efficiency, the emerging High-Efficiency Video Coding (HEVC) standard exploits block-based spatial prediction extensively. In HEVC, multiple Intra prediction modes are used to exploit spatial features and the number of Intra prediction modes depends on the block size of a Prediction Unit (PU). In the HEVC standard, angular Intra prediction supports 33 distinct prediction directions along with a Planar mode (i.e., mode 0) and a DC mode (i.e., mode 1) as shown in FIG. 1. The mode numbers from 2 through 34 are associated with angular prediction. Mode 2 through mode 18 cover prediction directions from −135° to −45°, where mode 2 corresponds to −135° direction, mode 18 corresponds to −45° direction and mode 10 corresponds to −90° direction. Mode 18 through mode 34 cover prediction directions from −45° to +45°, where mode 26 corresponds to 0° direction and mode 34 corresponds +45° direction. There are four Intra prediction block sizes including 4×4, 8×8, 16×16 and 32×32. The decoder supports 132 combinations of block size and prediction direction.
HEVC adopts Intra prediction mode-dependent coefficient scanning (MDCS) for 4×4 and 8×8 transform blocks, where the scanning order of transform coefficients is determined by the Intra prediction mode. The scanning patterns for an 8×8 transform block, including diagonal coefficient scanning, horizontal coefficient scanning and vertical coefficient scanning, are shown in FIG. 2. For Intra prediction angular modes 6-14, vertical coefficient scanning is applied to the underlying square transform block. For Intra prediction angular modes 22-30, horizontal coefficient scanning is applied to the underlying square transform block. For the rest modes, diagonal coefficient scanning is applied. FIG. 3 illustrates the mode-dependent (or mode-derived) coefficient scanning patterns for an 8×8 transform block according to the conventional MDCS of the HEVC standard, where modes in range 310 use vertical coefficient scanning, modes in range 320 use horizontal coefficient scanning, and the rest modes use diagonal coefficient scanning.
In HEVC Range Extension, 4:2:2 color subsampling pattern is supported, where a chroma block collocated with a 2N×2N luma block has a size N×2N. The angular Intra modes for an N×2N chroma block are shown in FIG. 4, where the Intra modes for the non-square block are derived by mapping the Intra modes for the square block to the Intra modes (named mapped Intra modes) for the non-square block. The Intra prediction MDCS for the chroma components is the same as that for the luma component. As shown in FIG. 4, mapped Intra prediction angular modes 6-14 for the chroma components cover a much wider angle 410 than Intra prediction angular modes 22-30 for the luma component 420. This also applies to general N×2N luma as well as chroma intra block prediction. In one example, a 2N×2N intra coding unit (CU) may be split into two N×2N prediction units. In some cases, a 2N×N prediction block may also be derived from a 2N×2N coding or prediction block. For example, a 2N×N block may be generated from a 2N×2N block in a scalable coding system, where 2:1 scaling is performed in the vertical direction. In another example, a 2N×2N intra coding unit (CU) is split into two 2N×N intra prediction units. The MDCS method may be applied to the non-square blocks in the scalable coding system as shown in FIG. 5. As shown in FIG. 5, mapped Intra prediction angular modes 6-14 for the chroma components cover a much narrower angle 510 than Intra prediction angular modes 22-30 for the luma component 520. The original MDCS design is tailored for the Intra predictive coding of luma component with square block sizes. Therefore, if the same MDCS design is applied to the non-square blocks of chroma components, the performance may be degraded. Therefore, it is desirable develop Intra coding suited for non-square blocks.